A fixed print contains a considerable amount of thiosulfate, which must be removed to optimize the longevity of the silver image.
The process of print washing is a combination of displacement and diffusion. Just prior to the wash, a relatively large amount of excess fixer is gently clinging to the print through surface adhesion. An initial, brief but rapid, rinse in water quickly displaces this excess fixer, simply washing it off the surface. However, there is still plenty of thiosulfate left in the print, and this is a bit harder to get rid of. It has been deeply absorbed by the emulsion and saturates the print fibers. The remaining thiosulfate can only be removed by the process of diffusion.
As long as there is a difference in thiosulfate concentration between the print and the wash water, thiosulfate will diffuse from the print into the water. This gradually reduces the thiosulfate concentration in the print and increases it in the wash water. Diffusion continues until both are of the same concentration and an equilibrium is reached, at which point, no further diffusion takes place.
Replacing the saturated wash water entirely with fresh water repeats the process, and a new equilibrium at a lower residual thiosulfate level is obtained. However, diffusion is an exponential process that decreases geometrically with time. This means that the rate of diffusion slows down rapidly towards the equilibrium. Print washing is quicker if the wash water is not entirely replaced in certain intervals, but slowly displaced with a constant flow of fresh water across the print surfaces, keeping the concentration difference, and therefore the rate of diffusion, at a maximum during the entire wash.
Other essential elements for effective washing are the use of washing aid, water replenishment and temperature.
useful explanation and extra insight Ralph. Makes a lot of sense.
Thanks to all for your thoughtful replies. Rethinking my own process, hoping to be more efficient with water, is what got me looking into the paper-washing process again.
It seems that a condensed thread on effective, print washing methods would be very helpful as a sticky. Some measurable variables could include time for effective wash, and amount of water used for print.
According to Cole Weston in Darkroom 2, his father used a Kodak Tray Siphon to wash his prints. My old professor told us that if we couldn't get a washer, but wanted to do work at home we could wash by filling a tray with water, shuffling the prints continuously for 5 minutes, then dumping the water out, refilling and shuffling for another 5 minutes, and so on for a total of one hour. This would use 12 trays worth of water. At 1/2 gallon per tray for an 11x14 tray and 6-12 prints you would use 6 gallons of water to wash. This is for fiber paper. RC would be fine with 3 cycles of water instead of 12.
Greg, thanks for letting me know that Edward used the Kodak tray siphon.
I am getting some silver nitrate to make the HT-2 residual hypo test, and plan to test out a couple of methods. One will be the method outlined by your teacher. Thanks for passing that along.
Sponsored Ad. (Subscribers to APUG have the option to remove this ad.)
Unbelievable. A ridiculous, absurd amount of work, time,
Originally Posted by Greg Davis
and water. Enough to discourage any one from taking up
darkroom photography. Twelve changes of water and
twixt changes, shuffle, shuffle, shuffle, then shuffle
ONE long soak can produce prints which are entirely
stainless by the HT-2 test. I've had prints entirely clean
of stain after ONE long soak. Clean sooner than I had
I've decided that such effortless water stingy results are do
to the very dilute one-shot fix which precedes a hold
and soak. A long 12 plus hour soak follows.
The method is not completely work free or time required none.
The water from the hold-soak tray, once the last print has had
a few minutes, is poured into a second tray and stirred. The
prints and separators are then placed in the second tray.
The top separator is placed on the bottom of tray 2.
And so tray 1 empties top to bottom and tray 2
builds bottom to top. The prints should have
30 to 60 minute of soaking in tray 2. After
they are ready for the long soak.
The long soak is a repeat of the hold-soak sequence save for
the amount of soak time; from hold-soak tray 2 to tray 1, the
first of the long soak trays. I usually make a transfer to tray
2 an hour or so before removing the prints. They are then
sponge dried and racked.
It's a least hassle, least water, least time consuming way to
wash. Those that use fixer in a customary way should follow
the fix with a rinse, a hca, a rinse then the above
outlined method. Dan
Tray washing is the simple effective way to wash prints. A tray wash is the low cost, water efficent and fastest method. Use an alkaline fix at film strength, fix for 60s, discard the fix after 10-12 8x10s per L. No need to use a hypo clear with a fix like TF4, unless you tone in selinium. If toned, process in hypo clear for 10 min.
Tray wash for 20-30 min with intermintent agitation, filling and dumping water in the tray 6 or 7 times. Do not let the prints overlap/stick or load up the tray with prints. (3) 8x10s in a 12x16 inch tray is about right.
I watch CNN while washing. Who says guys can't multitask as long as the tasks are simple.